David Westin announced Monday night that he is resigning as president of ABC News after a tough year that included anchor changes on every broadcast and cutting a quarter of the staff.

Thirteen years after he succeeded Roone Arledge, Westin, 58, felt he was ready for another career and in May told Bob Iger, chief executive of Disney, the network's parent company, that he wanted to wrap up his tenure. Westin will stay on the job until year's end to give Disney time to find a successor.

"I've always admired those few who know when it's time to move on," Westin said in a letter to the staff. "This is the right time for me."

In a series of personnel decisions that tumbled like dominos, Westin named Diane Sawyer the anchor of "World News"; installed George Stephanopoulos in her old job as co-host of "Good Morning America"; brought in Christiane Amanpour for the Sunday program "This Week"; sent Chris Cuomo from "GMA" to "20/20"; and put Bill Weir on "Nightline," replacing Martin Bashir.

"Leading you has been a great privilege and a solemn responsibility -- a responsibility that I tried to fulfill for over thirteen years by doing what I believed was best for this important news organization," Westin told the staff.

At the same time, he wrote, "there are some other things I want to do professionally -- things that I cannot explore while fulfilling my responsibilities here." Westin is said to be thinking of doing some writing and speaking; he had to kill a proposed op-ed piece on the Supreme Court after his advisers told him it was too opinionated for a network news leader.

In a note to the staff, ABC President Anne Sweeney said that "David proved himself a tireless advocate for ABC News, effectively guiding the group through some of the most seismic industry, and divisional, changes imaginable" and "helped reinvent our news organization." She said she will announce a successor "in the near future."

Westin had to deal with a number of calamities during his tenure. After Peter Jennings died in 2005, Westin tapped Bob Woodruff and Elizabeth Vargas for "World News," but he was badly injured by a bomb in Iraq and she became pregnant. Westin moved Vargas to "20/20" in favor of Charlie Gibson. He also had to manage the "Nightline" transition from Ted Koppel, who helped found the broadcast in 1979, to a three-anchor team.

But the layoffs -- which Westin called "a very difficult transformation made necessary by changes in our business and its economics" -- were perhaps the most wrenching set of decisions, as many veteran correspondents and producers were cut loose in an effort to shrink the network's costs.

Westin's promotion to the top job in 1997 was greeted by skepticism in some quarters because he is a lawyer, not a journalist, by training. And he succeeded Arledge, whom Westin himself called a "legend" and who in the previous two decades transformed both ABC's news and sports divisions from a backwater to a powerhouse on par with CBS and NBC.

It was the free-spending Arledge who attracted such stars as Sawyer, Koppel and David Brinkley and created such programs as "Nightline" and "This Week." It took time for Westin to emerge from that considerable shadow.

There were some early missteps, such as sending Leonardo DiCaprio to interview President Bill Clinton for Earth Day and a comment after the Sept. 11 attacks, for which Westin apologized, that journalists should offer no opinion about whether the Pentagon had been a legitimate military target. But Westin championed the news division in 2002 when the network tried to lure David Letterman from CBS to replace "Nightline," a deal that soon fell apart.

With his smooth style and friendly relations with reporters, Westin grew more comfortable in his role and is now the longest-serving network news president. He managed to massage the considerable egos at ABC: moving first Gibson and then Sawyer into the anchor job, maintaining a role for Barbara Walters until her recent retirement, and helping Stephanopoulos make the transformation from White House operative to morning star.

Despite occasional rumors that the bosses in Los Angeles were unhappy with him, Westin proved to be a survivor -- one who got to announce his departure on his own terms.